In a digital communication network, such as a telecommunications network or a private network having several private branch exchanges, it is often necessary to synchronise the time clocks of respective network nodes in order to ensure correct operation of the network. Network synchronisation permits all nodes on the network to operate from a common time base. This means that when one node (i.e. an intersection point) sends data to another node, both nodes can be expected to operate at approximately the same rate ensuring the successful transfer of data between the nodes. Background information on the need for network node synchronisation can be found in EP0450828.
In so-called “master-slave” synchronisation, one master node is chosen to distribute high quality clock signals (generated by a Primary Reference Clock (PRC)) to all slave nodes in a hierarchy of network nodes. The master node distributes PRC clock signals to adjacent nodes which in turn distribute the received and regenerated clock signals to their adjacent nodes until all the nodes in the network are using the same clock origin.
The need for synchronisation is especially important in mobile telecommunication networks, and will become even more so with the introduction of Universal Mobile Telecommunications System (UMTS) networks where the UMTS Terrestrial Radio Access Network (UTRAN) places very severe limits on network synchronisation.
A typical UTRAN configuration consists of Radio Network Controllers (RNCs) which perform switching functions in the network (analogous in some ways with conventional telephone exchanges and with Mobile Switching Centres of GSM networks) and Radio Base Stations (RBSs) which provide the interface between the UTRAN and the mobile terminals (each RBS being responsible for a given cell). The RNCs and RBSs are arranged in a hierarchy (or hierarchies) with a single RNC possibly being responsible for tens of RBSs. The link structure in a UTRAN may be complex, with nodes of the same type being linked to one another as well as to nodes of a different type. In certain circumstances, synchronisation may be taken from a co-located GSM network or UTRAN synchronisation may be utilised in GSM nodes.
In the event of a synchronisation failure, e.g. due to the failure of a link between two nodes, action must be taken quickly to re-establish synchronisation. This usually means selecting for the node suffering from the effects of the failure (as well as for other nodes downstream of that node) an alternative incoming link which can be used to achieve synchronisation. Typically, certain incoming links are preferred to other links for this purpose, and the selection of an appropriate link requires a network level administration system which is connected to all network nodes. This work requires each node of the network to have a complete knowledge of the network and, in failure situations, the network synchronisation can suffer from unforeseen combinations of the network nodes.
WO95/24801 describes a method of synchronising a network by propagating synchronisation messages down through a hierarchy of network nodes. The synchronisation messages each comprise a master node address, a distance-to-master node, indicated as the number of intermediate nodes through which the message has passed, and the identity of the transmitting node. Each node through which a message passes, increases a distance counter by 1 and changes the transmitting node identity to its own identity. The path field allows receiving nodes to prioritise incoming links for synchronisation purposes.
WO96/39760 describes a method of detecting timing loops in a Synchronous Digital Hierarchy (SDH) network by sending a synchronisation message consisting of the identities of all the nodes through which the synchronisation message has passed. The synchronisation message also contains a count of the number of nodes through which clock signal has passed. This is used to prevent excessively long synchronisation chains.